The present invention relates to a process for preparing aqueous, radioactive waste solutions, from reprocessing plants for spent nuclear fuel and/or breeder materials and other nuclear plants, for noncontaminating solidification and/or removal of such solutions, and more particularly relates to a process in which the total quantity of the various inorganic and organic substances contained in the aqueous radioactive waste solutions are reduced by destroying nitric acid, nitrates and nitrites and forming a waste gas mixture which is practically free of higher nitrous oxides.
Radioactive waste, no matter what kind, must be permanently stored or removed, respectively, in a manner such that there practically can be no possibility of discharge of dangerous radionuclides into the biocycle, even under accidental, or unfavorable conditions, for example, at the final storage location.
In addition to providing a safe final storage location, additional measures must be taken to insure safe handling of the aqueous radioactive solutions before they reach a final storage location and to further insure that the radioactive wastes are safe at the final storage location. For this reason, dangerous radionuclides, before they are transported and/or permanently stored or removed, must be brought into a form which substantially prevents the undesirable release or distribution of these radionuclides into the biocycle before they have lost their dangerous properties or have been converted to stable, harmless nuclides. In order to bring radionuclides into such a form, solidification processes are used, such as, for example, fixing them in glass, ceramic, or basalt-like masses, bitumen, cement rock, and the like. The products from such processes are shaped bodies, with or without protective sheathings, or masses which are introduced into drums or similar containers during the process and solidify therein. The volumes of the shaped bodies and of the solidified masses should be kept as low as possible.
Aqueous, radioactive waste solutions from nuclear plants must thus be prepared for solidification. For this purpose, particularly if they are highly radioactive, the aqueous radioactive waste solutions are initially concentrated in an evaporator and the concentrate, which is 4 to 6 molar nitric acid, is intermediately stored for several years in expensive, cooled and ventilated container systems of stainless steel until part of the radioactivity has decayed. In order to reduce the rate of corrosion of the stainless steel, it is the custom to either partially neutralize the nitric acid or to add reduction agents such as sugar, formaldehyde or formic acid, to reduce the nitric acid concentration in the concentrate to 1 to 2 mols/liter. This procedure is expensive and the use of reduction agents involves a certain risk because such reactions sometimes may take place at an uncontrollable speed.
In the case where formaldehyde solutions are used, the reaction is difficult to control, foam forms and the formaldehyde may polymerize. If sugar solutions are used, foaming is also a problem and an excess of sugar may lead to explosions when the dry residue is heated.
Reactions of nitric acid with formaldehyde and with formic acid, in theory and in practice, are described by T. V. Healy in the Journal of Applied Chemistry, Vol. 8 (September, 1958), on pages 553-561. In his practical experiments, the reduction agent was always introduced into the nitric acid. This produced waste gas mixtures which contained considerable proportions of higher nitrous oxides. The composition of the waste gas mixtures lay in the range of the two compositions described below. The first composition contained, in volume percent,
4% NO.sub.2, 36% NO, and 60% CO.sub.2. PA1 82% NO.sub.2, 0% NO, and 16% CO.sub.2. PA1 78% CO.sub.2, 16% N.sub.2 O, 5% NO, and 1% N.sub.2. PA1 No.sub.2, hno.sub.2, n.sub.2 o, no, n.sub.2, nh.sub.2 oh and NH.sub.4.sup.+.
The second composition contained, in volume percent,
In another process for removing nitric acid and/or nitrate and nitrite ions from aqueous waste solutions, as described in U.S. Pat. No. 3,673,086 to Drobnik, the originally present free nitric acid, all nitrite ions, and all of the nitrate ions belonging to the more than two-valent cations and to all heavy metal cations are completely destroyed upon the controlled introduction of the waste solution into nitric acid or a nitric acid solution. A waste gas mixture is obtained which is independent of the acid concentration of the starting solution. The gas mixture is practically free of higher nitrous oxides and has the following composition, by volume:
Highly radioactive, aqueous waste solutions (HAW) are produced in the first extraction cycle of the presently employed reprocessing procedures for spent nuclear fuel and/or breeder materials according to the Purex type (plutonium reduction and extraction). These highly radioactive waste solutions are introduced, after intermediate storage, as a liquid, or by means of a Thermit process, to solidify in glass, ceramic or basalt-like masses. This, however, makes it absolutely necessary that the waste solutions be previously subjected to a substantial denitration with subsequent calcination.
In order to reduce the long-term damage potential during permanent storage of solidified HAW's, the necessity of separating the long-lived transuraniums from the HAW's before solidification is presently being discussed with increasing fervor. Proposed processes with this in mind always include denitration of the liquid HAW to a pH between 2 and 5, and selective extraction of the actinides with strong complex formers such as, for example, di-(2-ethylhexyl)-phosphorus acid (HDEHP) or others.
The solutions obtained in the subsequent extraction cycles and at other points in the Purex process contain smaller amounts of fission products and residual fission products, and are considered as medium active wastes (MAW). These medium active waste solutions must also be separated from free nitric acid before they are solidified by, for example, bituminization or embedding in cement. This is presently frequently done by neutralization with sodium liquor or also by the addition of reduction agents.
Difficulties often arise during denitration with chemical reduction agents, inter alia, during the exact setting of a pH within the above-mentioned narrow range.
The cathodic reduction of nitric acid to nitrous acid is known and the reaction sequence has been discussed. See, for example, G. Schmid, Magazine for Electrochemistry, volume 63 (1959), issue 9/10, pages 1183-1188). The lowest oxidation stage of the reduction products mentioned therein, however, is nitrous oxide (N0).
In USAEC Report No. KAPL-1263, "Investigation of Electrolysis as a Method for the Treatment of Radioactive Nitric Acid Wastes," January 4, 1955 (Knolls Atomic Power Laboratory), D. L. Barney reports, however, that the following substances have been identified as products of electrolytic reduction of nitric acid under various conditions:
The report states that the predominant product is nitrogen dioxide. NO.sub.2 and NO are desirable because of their easy recombination with anodically produced oxygen and with water to form HNO.sub.3, while N.sub.2 O, N.sub.2 and the NH.sub.4.sup.+ ions are undesirable. It is further stated in the report that during the electrolytic reduction of HNO.sub.3, hydrogen is formed first, followed by a period of nitric acid reduction in which practically no H.sub.2 is produced until the HNO.sub.3 concentration has been reduced to a certain critical value. Below this critical HNO.sub.3 concentration, H.sub.2 is again the predominant reduction product. D. L. Barney used platinum electrodes and, under various conditions, found values for the critical HNO.sub.3 concentration which never fell below the value of 3 mols of HNO.sub.3 per liter. At a current density of 840 mA/cm.sup.2 and a potential of 4.1 V, the critical concentration was about 5.9 mols of HNO.sub.3 per liter. At a current density of 280 mA/cm.sup.2 and a potential of 1.9 V, the critical concentration was about 3.8 mols of HNO.sub.3 per liter.
In the Barney report, N.sub.2 O was produced below the critical concentration, but before H.sub.2 constituted the sole electrolysis product. As shown in the graphic illustrations in the Barney report regarding the composition of the waste gas in dependence on the HNO.sub.3 concentration, no N.sub.2 O is produced below a concentration of 3 mols of HNO.sub.3 per liter. Only H.sub.2 is produced below this concentration. Barney further reports of tests with an addition of 0.01 M Cu(NO.sub.3).sub.2 in which, in spite of an improved HNO.sub.3 reduction, residual concentrations of HNO.sub.3 remained. On the other hand, as evidenced by the composition of the waste gases, the formation of NO.sub.2 could not be completely avoided. Upon comparing the electrolytic reduction with the acid distillation, Barney came to the conclusion that acid distillation was more attractive.